I had already pre-ordered the lens mid-May at Amazon. Negative surprise a few days ago. Amazon reports on the website a delivery time of 1-3 months. A day later I received more surprising a shipping confirmation by email. Since yesterday I am now the proud owner of 16-35 4.0. I was able to try out a bit the lens. It is to, together with the EOS 6D well in the hand and is about the same size and weight, we the 24-105 Canon. The autofocus (I use only the central sensor) also speaks in very low light (only desk lamp in the room) to very good, significantly better than the 24-105. I also shot a few pictures from the tripod (the IS recognizes the tripod easily) to judge the sharpness. Even at full aperture is reflected across all focal lengths uniform sharpness from the center to the extreme edges. Fade hardly improves the sharpness results. However, the contrast is still increasing slightly. I've done out of curiosity and a comparison at 24mm with my 24 1.4L II and could actually find only homeopathic differences. The margins are at 24 1.4L II to Aperture 5.6 !! weaker. However, the strengths of the lens are in another field for me (Night, Low Light, Street). Go back to 16-35. I suspect it is the medium 17-40 outdo because it most photographers but offers more in the sum of the properties. Feather (I'm mainly Landschaf photographer without a tripod and appreciate that already) and the much-discussed IS at a wide-angle lens. Especially on vacation, there are countless situations in which one drags a tripod and predominantly static subjects still would like to take freehand without high ISO. And that succeeds perfectly. Should I still make more valuable experience with the lens, I will write an update here. As far as my first impression.